The 60 GHz band is an unlicensed band which features a large amount of bandwidth and a large worldwide overlap. The large bandwidth means that a very high volume of information can be transmitted wirelessly. As a result, multiple applications that require transmission of a large amount of data can be developed to allow wireless communication around the 60 GHz band. Examples of such applications include, but are not limited to, wireless high definition TV (HDTV), wireless docking stations, wireless Gigabit Ethernet, and many others. Wireless local area network (WLAN) standards, such as WiGig Alliance (WGA) and IEEE 802.11ad, are being developed to serve applications that utilize the 60 GHz spectrum.
Such communication standards enable wireless transmission between two stations that are a short distance from each other. Typically, in such wireless transmission systems, signals circulate between transmitters and receivers by way of channels. Due to many factors in channel characteristics, an unwanted distortion is induced in the signal transmitted by the transmitter. Accordingly, it is generally necessary to determine the characteristics of a channel at a given moment in order to estimate the induced distortion in the transmitted signal.
There are a number of techniques for performing channel estimation in wireless transmission systems. For example, one technique includes transmitting signals with predetermined sequences via a transmitter and then comparing the signals received in a receiver using auto-correlation and cross-correlation with expected signals in order to estimate the characteristics of the channel. The sequences of the transmitted signals are known to the receiver. The results of the correlation operation constitute the estimate of the impulse response of the channel.
In millimeter-wave wireless transmission systems operating in the 60 GHz band, for example, as defined by the IEEE 802.11ad standard published Dec. 28, 2012 (hereinafter the IEEE 802.11ad standard), a preamble is employed as part of each physical layer convergence procedure (PLOP) protocol data unit (PPDU) used for channel estimation. The preamble is commonly used for both orthogonal frequency-division multiplexing (OFDM) packets and single carrier (SC) Packets.
As shown in FIG. 1, an IEEE 802.11ad preamble 100 is composed of two parts, a short training field (STF) 120 and a channel estimation field (CEF) 130. Both the STF 120 and CEF 130 fields contain Golay complementary sequences, which are transmitted by a transmitter and are auto-correlated by a receiver in a millimeter-wave wireless transmission system. Typical Golay complementary sequences have many advantageous properties, such as producing a perfect sum of autocorrelations and providing efficient implementations requiring only log2(N) additions for two complementary sequences of length N.
As shown in FIG. 1, the STF 120 is composed of 16 repetitions of the sequence Ga128(n) in length 128 followed by a single repetition of -Ga128(n). The CEF 130 is used for channel estimation, as well as for indicating which modulation is going to be used for the packet. The CEF 130 is composed of a concatenation of two sequences, Gu512(n) and Gv512(n), wherein the last 128 samples of Gu512(n) and Gv512(n) are equal to the last 128 samples used in the short training field (e.g., -Ga128(n)). These sequences are followed by a 128 sample sequence, Gv128(n), equal to the first 128 samples of both Gu512(n) and Gv512(n) (e.g., -Gb128(n)). In total, the CEF 130 includes a channel estimate of 1152 symbols while the STF 120 includes 2176 symbols.
The precision of the channel estimation at the receiver is determined by the signal-to-noise ratio (SNR) at the receiver. The SNR is an indicator often used to evaluate the quality of a communication link. For example, according to the IEEE 802.11ad standard, the SNR measured during the reception of a control PHY packet can be valued from −13 dB to 50.75 dB in 0.25 dB steps. As indicated above, the 802.11ad channel estimation sequence is 1152 symbols long, which yields an estimated channel of length 128, thereby resulting in a precision value of SNR+9 dB.
The resulting channel estimation is fed directly into an equalizer or into a frequency domain transform such as a fast Fourier transform (FFT) for further processing that does not improve the precision of the channel estimation. As alluded to above, the purpose of the channel estimation is to obtain high performance in the presence of a very dispersive channel with several echoes by using one or more equalizers to mitigate inter-symbol-interference (ISI) caused by the channel. In order for the equalizers to work properly, they require a good channel estimate that models the ISI effect as a linear filter response.
Hence, there is a need for a solution to enhance the channel estimation in a millimeter-wave wireless transmission system.